1. Field of the Invention
This invention relates to manufacture of ribbon or wire by rapid quenching of a molten alloy, and more particularly to compositional and structural characteristics of a casting wheel substrate used to obtain the rapid quench.
2. Description of the Prior Art
Continuous casting of alloy strip is accomplished by depositing molten alloy onto a rotating casting wheel. Strip forms as the molten alloy stream is maintained and solidified through conduction of heat by the casting wheel""s rapidly moving quench surface. The solidified strip departs the chill wheel and is handled by winding machinery. For continuous casting of high quality strips, this quenching surface must withstand thermally generated mechanical stresses due to the cyclic molten metal contact and removal of solidified strip from the casting surface. Any defect in the quenching surface is subject to penetration by the molten metal, whereupon the removal of solidified strip plucks away portions of the chill surface causing further degradation of the chill surface. As a result, the surface quality of the strips suffers as longer lengths of strips are cast within a given track on a chill wheel. The cast length of high quality strip provides a direct measure of the quality of the wheel material.
Key factors for improved performance of the quench surface are (i) use of alloys having high thermal conductivity, so that heat from the molten metal can be extracted to solidify the strip and (ii) use of materials with high mechanical strength to maintain the integrity of the casting surface, which is subjected to high stress levels at elevated temperature ( greater than 500 C). Alloys that have high thermal conductivity do not have high mechanical strength, especially at elevated temperatures. Therefore, thermal conductivity is compromised to use alloys with adequate strength characteristics. Pure copper has very good thermal conductivity, but shows severe wheel damage after casting short lengths of strip. Examples include copper alloys of various kinds and the like. Alternatively, various surfaces can be plated onto the casting wheel quench surface in order to improve its performance, as disclosed in European Patent No. EP0024506. A suitable casting procedure has been described in detail by U.S. Pat. No. 4,142,571, the disclosure of which is incorporated herein by reference.
Casting wheel quench surfaces of the prior art generally involve one of two forms: monolithic or multi-component. In the former, a solid block of alloy is fashioned into the form of a casting wheel that is optionally provided with cooling channels. Component quench surfaces comprise a plurality of pieces which, when assembled, constitute a casting wheel, as disclosed in U.S. Pat. No. 4,537,239. The casting wheel quench surface improvements of the present disclosure are applicable to all kinds of casting wheels.
Casting wheel quench surfaces have conventionally been made from a single-phase copper alloy or from a single-phase copper alloy with coherent or semi-coherent precipitates. The alloy is cast and mechanically worked in some manner prior to fabricating a wheel quench surface therefrom. Certain mechanical properties such as hardness, tensile and yield strength, and elongation have been considered, in combination with compromises to thermal conductivity. This has been done in an effort to achieve the best combination of mechanical strength and thermal conductivity properties possible for a given alloy. The reason for this is basically twofold: 1) to provide a quench rate which is high enough to result in the cast strip microstructure which is desired, 2) to resist quench surface thermal and mechanical damage which would result in degradation of strip geometric definition and thereby render the cast product unusable. Typical alloys exhibiting a single phase with coherent or semi-coherent precipitates include copper beryllium alloys of various compositions and copper chromium alloys with low concentrations of chromium. Both beryllium and chromium have very little solid solubility in copper.
The strip casting process is complicated and dynamic or cyclical mechanical properties need to be seriously considered in order to develop a quench surface that has superior performance characteristics. The processes by which the feedstock single-phase alloy for use as a quenching surface is made can significantly affect subsequent strip casting performance. This can be due to the amount of mechanical work and subsequent strengthening phases which occur after heat treatment. It can also be due to the directionality or the discrete nature of some mechanical working processes. For example, ring forging and extrusion both impart anisotropy of mechanical properties to a work piece. Unfortunately, the direction of this resulting orientation is not typically aligned along the most useful direction within the quench surface. The heat treatment employed to achieve alloy recrystallization and grain growth and strengthening coherent phase precipitation with the single phase alloy matrix is often insufficient to ameliorate the deficiencies induced during the mechanical working process steps. The resultant quench surface exhibits a microstructure having non-uniform grain size, shape, and distribution. Changes in the processing of these single phase copper alloys, which have been used to obtain uniform fine equiaxial grain structure are disclosed in U.S. Pat. Nos. 5,564,490 and 5,842,511. The fine grained homogenous single phase structure reduces formation of large pits in the casting wheel surface. These pits, in turn, create corresponding xe2x80x98pipsxe2x80x99 in the strip surface that contacts the wheel during the casting process. Many of these precipitation hardenable single phase copper alloys contain beryllium as one of their components. The biological toxicity aspects of a beryllium containing alloy, which is constantly polished to improve the quality of the casting surface, poses a health risk. Accordingly, non-toxic alloys that exhibit good molten metal quenching properties without surface degradation have been long sought.
Copper-nickel-silicon alloys with other elemental additions have been used as a replacement for beryllium copper alloys in the electronic industry, as disclosed in U.S. Pat. No. 5,846,346. The precipitation of second phase is suppressed to provide high thermal conductivity and strength. Japanese patent publication number S60-45696 suggests adding 14 additives to produce very fine precipitates in certain Corson group alloys. These essentially single-phase alloys contain Cu with 0.5 to about 4 wt % Ni and 0.1 to about 1 wt % Si. Casting temperature capabilities for this essentially single-phase alloy are well below the requirements of a rapid-quench casting surface.
As a consequence remains a need in the art for non-toxic chill wheels for rapid solidification of molten alloy, which retain the surface quality of cast strips by resisting rapid deterioration during casting for a prolonged period of time. This need has heretofore not been met by existing essentially single-phase copper alloys even when the grain structure is well controlled.
The present invention provides an apparatus for continuous casting of alloy strip. Generally stated, the apparatus has a casting wheel comprising a rapidly moving quench surface that cools a molten alloy layer deposited thereon for rapid solidification into a continuous alloy strip. The quench surface is composed of a two-phase copper-nickel-silicon alloy having minor additions of other elements.
Generally stated, the alloy has a composition consisting essentially of about 6-8 wt % nickel, about 1-2 wt % silicon, about 0.3-0.8 wt % chromium, the balance being copper and incidental impurities. Such an alloy has a microstructure containing fine grains of the copper phase surrounded by thin well-bonded network regions of nickel silicide. Alloys having this microstructure are processed using certain alloy-manufacturing casting and mechanical working methods, and final heat treatment. The microstructure of the alloy is responsible for its high thermal conductivity and high hardness and strength. The thermal conductivity is derived from the copper phase and the hardness is derived from the nickel silicide phase. Distribution of the surrounding network phase creates a cell structure with cell size in the 1-250 xcexcm range, presenting a substantially homogeneous quench surface to the molten melt. Such an alloy resists degradation during casting for a prolonged period of time. Long lengths of strips can be cast from such molten alloys without formation of surface projections known as xe2x80x98pipsxe2x80x99, or other surface degradation.
Generally stated, the quench casting wheel substrate of the present invention is produced by a process comprising the steps of: (a) casting a copper-nickel-silicon two phase alloy billet having a composition consisting essentially of about 6-8 wt % nickel, about 1-2 wt % silicon, about 0.3-0.8 wt % chromium, the balance being copper and incidental impurities; (b) mechanically working said billet to form a quench casting wheel substrate; and (c) heat treating said substrate to obtain a two-phase microstructure having a cell size ranging from about 1-1000 xcexcm.
Use of a two-phase crystalline quench substrate advantageously increases the service life of casting wheel. Run times for casts conducted on the quench surface are significantly lengthened, and the quantity of material cast during each run is improved without the toxicity encountered with copper-beryllium substrates. Strip cast on the quench surfaces exhibits far fewer surface defects, and hence, an increased pack factor (% lamination); the efficiencies of electrical power distribution transformers made from such strip are improved. Run response of the quench surface during casting is remarkably consistent from one cast to another, with the result that the run times of substantially the same duration are repeatable and scheduling of maintenance is facilitated. Advantageously, yields of strip rapidly solidified on such substrates are markedly improved, down time involved in maintenance of the substrates is minimized, and the reliability of the process is increased.